Fix some of the StrictMode violations (#1931)

Mostly it's disk reads/writes.

Conscrypt reads own version on startup which reads from disk multiple
times. There's no solution for it right now.

SharedPreferences which are used in BaseActivity also read from disk
and pretty early but it shouldn't be a problem.

app/src/main/java/com/keylesspalace/tusky/MainActivity.kt

@@ -81,6 +81,7 @@ import com.uber.autodispose.android.lifecycle.autoDispose
 import dagger.android.DispatchingAndroidInjector
 import dagger.android.HasAndroidInjector
 import io.reactivex.android.schedulers.AndroidSchedulers
+import io.reactivex.schedulers.Schedulers
 import kotlinx.android.synthetic.main.activity_main.*
 import javax.inject.Inject
 
@@ -227,8 +228,10 @@ class MainActivity : BottomSheetActivity(), ActionButtonActivity, HasAndroidInje
                     }
                 }
 
-        // Flush old media that was cached for sharing
-        deleteStaleCachedMedia(applicationContext.getExternalFilesDir("Tusky"))
+        Schedulers.io().scheduleDirect {
+            // Flush old media that was cached for sharing
+            deleteStaleCachedMedia(applicationContext.getExternalFilesDir("Tusky"))
+        }
     }
 
     override fun onResume() {